With its signature hump, the world's fastest subsonic jetliner cruises at 565 m.p.h., or about 85 percent of the speed of sound at 30,000 feet. To the 10,200 people working in Sears Tower, the jumbo jet approaching from the north Sept. 20 was as tiny and unknowable as a germ. But soon it would torment the overworked imaginations of hundreds still skittish after what happened in New York nine days before.

A few minutes before 10:30 a.m., as Flight 945 closed fast on Chicago, a flight attendant told the pilot that a woman onboard was having chest pains.

Reaching for the transponder box, either the pilot or the co-pilot dialed in the four-digit code for an emergency so the control tower would know.

But the second digit came out wrong. The code should have been 7700. Instead it came out 7500, sending the erroneous signal that a hijacker had taken over the plane and setting in motion a chain of events that quickly traversed hundreds of miles.

Before long a trader for Merrill Lynch on the floor in New York overheard a conversation on a speakerphone--a call, authorities later would say, from a passenger on Flight 945. It sounded to the eavesdropping trader like the passenger was saying the plane had been hijacked in Milwaukee and was headed for Chicago.

Outside New York and Washington and far from any field of combat, the battle joined Sept. 11 was, for most, a fierce inner struggle.

Inside the nation's tallest skyscraper, innumerable personal dramas played out, as companies pondered finding new homes and the tower's managers scrambled to make the tower more secure. This premier address, this architectural marvel with spectacular views, had become the site of a distorted reality in which lower floors were coveted and some employees prayed for layoffs. Over time, workers there, like the rest of the country, found ways to cope and settled into a new kind of normalcy.

For some of them, the hijacking scare of Sept. 20 would prove to be a turning point, informing life-changing decisions. It would persuade a securities broker on the eighth floor to keep working in the building while giving a research associate on the 70th floor one more reason to leave.

Nancy Dombrowski, a securities broker at Stifel, Nicolaus & Co. Inc., was preparing to go downstairs for a smoke when her phone rang. It was a friend and former co-worker, a stock trader at 135 S. LaSalle St.

"Did you guys hear an alarm or get an evacuation announcement?" she recalls him asking.

"What are you talking about?" Dombrowski said.

"My buddy over there, he just called saying they're evacuating because of a plane hijacking out of Milwaukee."

"No," Dombrowski said, her stomach closing into a knot.

Putting the receiver on her shoulder, Dombrowski started gathering files, notebooks and client lists as she talked. Then she and her assistant, Dawn Farrell, bolted from the office they shared on the eighth floor.

The image of the World Trade Center's collapse haunted Dombrowski and Farrell, as it did many other people working in the tower and at nearby buildings.

Almost 1,000 workers spilled into the streets without being officially evacuated. Most were from the lower half of Sears Tower.

Farrell climbed into Dombrowski's white BMW convertible, and the two women roared up the Kennedy Expressway, top down. Wind lashed their blond hair in electric arcs--in their haste they had not taken the time to pull it back--and swept away the smoke from their Marlboro Lights.

Normally at this time of morning, they would be sitting at the counter in Ms. Levy's Delicatessen on the mezzanine while Farrell gnawed on "The Breakfast"--the generic name she used for ordering her favorite dish, extra-crispy bacon, hash browns and scrambled eggs with cheese, from the waitress who knew just what she meant.

Instead, they headed for Dombrowski's apartment in Wicker Park. They wanted to put some distance between themselves and Sears Tower. Dombrowski, half expecting to see the building sprout flames in her rearview mirror, felt nauseated.

They fled to her second-floor back deck, which afforded them a view of the tower in the distance.

Dombrowski and Farrell lit cigarettes and paced. They turned on CNN. They phoned the office. They kept returning to the eastern edge of the deck to study the dark tower.

Dombrowski said she thought, "I can't wait for all this to subside so that every day we're not just sick to our stomachs to go into that building."

It made her mad to be so scared.

Farrell, eyes wide, lit a cigarette, hands trembling, shoulders hunched slightly, as if against a chill nobody else could feel. She scissored the cigarette between her fingers, smoked it, lit another.

Smoked it. Lit another.

Smoked it. Lit another.

In the hour and a half they were gone from Sears Tower, Farrell smoked almost half a pack.

Carlos Villarreal, head of security at Sears Tower, phoned Sgt. Bill O'Reilly, the Chicago Police Department's liaison to more than three-dozen office buildings in the Loop.

O'Reilly was in a nearby high-rise talking to tenants to allay their fears about Sept. 11 when he got the call.

"There's a rumor that a plane is headed for Sears Tower," Villarreal told him.

Arriving at Sears Tower, O'Reilly saw something that struck him as odd: hundreds of people standing along Franklin Street, looking up.

It seemed like the last place any rational person would hang out, considering the nature of the rumor.

But rational thought was not in ready supply.

Starting about 11:30 a.m., several callers phoned the city's 911 center to report having heard that a hijacked plane was headed for Chicago. The last phoned at 12:10 p.m. from a building across the street from Sears Tower to request a police presence there.

By then Flight 945 had been on the ground for almost an hour and a half, having landed, without incident, at 10:45 a.m. at O'Hare International Airport--something police found out with a single call to the airport and announced on the street through megaphones and over the public-address systems of their cruisers.

Workers began streaming back to their offices.

On the 70th floor, about 15 employees of Heidrick & Struggles were holed up in the firm's long, white conference room for a two-hour group counseling session arranged to help them deal with their emotions after Sept. 11. They hadn't heard the hijacking rumor.

Some of them were angry about the terrorist attacks on New York and the Pentagon. Others were frightened. But many kept silent for fear of losing their jobs; the firm had laid off more than a dozen employees before Sept. 11, and rumor had it that more layoffs were coming.

The psychologist, who had an impressive resume, having counseled survivors of the Oklahoma City bombing, said everybody must realize there's danger in everything you do. Although he was fearful walking into Sears Tower, he said, he still did it.

"With all due respect," Judie Mikuzis, a research associate, told him, "you only have to walk in today. You don't have to come back tomorrow."

Mikuzis recalls that another woman chimed in, saying: "I have to walk in tomorrow. But I'm not afraid. I've had a good life, and if it's my time to go, it's my time to go."

Mikuzis turned to her.

"That's wonderful you have that feeling," she said. "But you're a little over 50. You've raised your children; you're comfortable where you are. I'm newly married. We want to start a family. I'm not ready to die."

When the session was almost two hours old, the public-address system clicked on with an electric buzz.

"All clear," a voice said.

All clear? All clear of what? Erin Frey wondered.

"See what we mean?" she asked, turning toward the psychologist.

The man looked rattled.

Erin Beavers thought, "I didn't even know we had a PA system."

The hijacking scare was neither the first false alarm nor the last. Between Sept. 11 and mid-October, the city's 911 center received hundreds of calls from or about Sears Tower.

Security officer Todd Marshall noticed many more people than usual taking pictures of the landmark in the weeks after Sept. 11. Some explained to him that they wanted to preserve an image of Sears Tower in case something happened to it.

One day a sports magazine that contained a powderlike substance feared to be anthrax brought veteran security officer Leroy Brown hurrying to the scene.

Another time it was a soggy box, delivered to the loading dock.

The box was about 1 1/2 feet deep and the size of standard printer paper. It was leaking and gave off a strange, overpowering odor. Brown cleared the area, pulled on a pair of rubber gloves from the blue pouch on his belt and wrapped the box carefully in cellophane until a hazardous-materials team from the city could arrive.

The odor turned out to be nothing more than ink that had gotten wet in transit and run all over the package, a box of promotional brochures that a firm had ordered. But that's the way it was. Construction noises, flickering lights--just about anything could be cause for alarm.

Messages from building management--which had resolved to let everyone know about every little thing that happened so nobody would panic--didn't always help. The e-mail alternated between disquieting and funny.

On Oct. 25 came this pre-Halloween message: "Sears Tower has elected to limit costume access to those that allow clear identification of the face and those that do not have large, concealed cavities."

Many people working in the tower dealt with the constant distraction of e-mail and scares by folding them into their daily routines with a dash of graveyard humor. Rick Schaschwary's colleagues on the 38th floor kidded him that he was on "plane watch" because his office in the southeast corner of the building afforded a breathtaking view of Lake Michigan. Schaschwary, a group-insurance policy writer, laughed nervously about that one.

Frey joked with co-workers and others about perceived security lapses, such as officers occasionally neglecting to search her bag, telling life-safety coordinator Mark Swiecionis, "I could be Saddam Hussein's girlfriend for all you know."

Leonor de la Torre, 46, a paralegal on the 81st floor, was concerned about the stairwells.

She didn't like how confusing taking the stairs is in Sears Tower. Because of the skyscraper's setbacks, only two stairwells run virtually top to bottom. The others break at various floors near the middle of the building, and de la Torre said it wasn't always easy to pick up the descent again.

De la Torre, a native of Colombia, had been a U.S. citizen since marrying an American more than 20 years earlier and moving to the United States so that their children could get a better education.

When her children grew up and returned to Colombia, de la Torre felt stuck and cut off from her family. But what happened Sept. 11 made her feel more like an American, in ways both good and bad.

De la Torre thinks Americans are openhearted and trusting; the terrorists took advantage of that. Paradoxically, she found herself feeling angry, even hateful, when she saw people who appeared to be of Middle Eastern descent--a feeling that made her almost as uncomfortable as working on the 81st floor.

Trizec Properties Inc., the company that manages Sears Tower, is in the midst of shelling out $5 million in security-related capital expenses, including reconfiguration of the lobby to accommodate new electronic turnstiles with identification-card readers.

The company hired armed, off-duty Chicago police officers to guard the lobbies from 6 a.m. to 6 p.m. Monday through Friday; set up X-ray machines and metal detectors in the lobby; put bomb-sniffing dogs to work on the Lower Wacker Drive service lane outside the loading dock to screen all delivery trucks before they can enter; and closed the Skydeck observatory for six weeks, reopening it with X-ray machines, metal detectors and off-duty Chicago police officers.

Because of the swooning economy and fears of another terrorist attack on skyscrapers and other landmark buildings, Trizec had trouble getting prospective tenants interested in any of its properties during the last quarter of 2001--especially in the skyscraper at 233 S. Wacker Drive.

"I'm routinely offering tenants to walk through Sears Tower, but since Sept. 11, I haven't shown one tenant the tower simply because they're not interested," says Michael Conway, president of Officedirectory.com, which helps firms find office space.

One client told Conway he didn't want space in Sears--or in any building within four blocks of it.

Although much of the initial fear has waned, many firms don't want to deal with the security hassles of a high-profile building, Conway says.

Since Sept. 11, the amount of rentable space leased in Sears Tower has remained more or less constant, at about 95 percent. But just because a space is under contract doesn't mean it is occupied or that its tenants plan to stay long. With many companies and firms in the tower locked into long-term leases, a more meaningful indicator of the skyscraper's comings and goings in the last year is the vacancy rate. Tenants seeking to leave a building on short notice often try to sublet their offices.

Currently, the combined vacancy rate, which includes sublease space, is 13.6 percent, up from less than 5 percent at the end of September 2001, according to CoStar Group, a Bethesda, Md.-based real estate research firm.

Trizec officials blame the jump in subleasing activity on the recession and a contraction in the telecom industry, which cost the tower one tenant leasing 136,000 square feet, or a fourth of the total vacant space.

Many firms have reduced or consolidated office space, a trend that took a toll on the entire downtown market. In fact, the overall vacancy rate for preferred modern office buildings in the West Loop area is higher than that at Sears Tower. The difference, brokers say, is that since Sept. 11, the tower's vacancy rate has risen faster.

Though Trizec had many of its tenants locked into long-term leases, a number of those expire in 2003 or 2004. A clue to what happens then might lie in Merrill Lynch & Co.'s decision that it will move part of its operation out of the tower when one of its leases ends next year.

The company won't comment on the move. Steve Budorick, Trizec's senior vice president, says Merrill Lynch needed less space but didn't want to endure the remodeling necessary to reconfigure its offices in Sears Tower. But at least one other planned departure appears driven largely by Sept. 11. General Reinsurance Corp. decided after Sept. 11 not to have offices in premier buildings, Budorick says.

Last winter, Trizec launched an advertising campaign that promotes Sears Tower as "A great place to office." The newspaper ads, which list amenities such as the executive health club and "unparalleled security," contain a curious omission:

They don't mention the world-famous views.

Psychologists and counselors came in waves to Sears Tower and nearby buildings.

Gloria Graff, a licensed clinical social worker with an office in the Loop, normally is called in by human resources directors about once a year for what's known as critical-incident stress management--group sessions to help people deal with company crises, such as layoffs or a colleague dying on the job.

After Sept. 11, Graff made five downtown office visits in three weeks, counseling workers whose greatest fear was that the nation's tallest building might become their tomb. Many were afraid that if Sears Tower fell it would take other buildings with it. Graff herself had calculated how far her 27th-floor office was from the tower; she took some comfort in being a dozen blocks away.

Three days after the terrorist attacks, she visited a large law firm with offices in a high-rise across the street from the tower. The emotional daytime session drew 50 people. Some cried. Others talked incessantly. Knowing the firm didn't want to pay for unlimited employee counseling, she ended the session after two hours, though many seemed to want to continue.

Graff returned to her office amazed. Imagine: She gave one of her workplace workshops at a law firm and lawyers actually came; usually it was all paralegals and secretaries.

It seemed Sept. 11 was all anyone thought about anymore. Graff's business was down. Through the middle of April, Americans seemed immobilized, depressed, sapped of energy, Graff said. For people to seek and undergo therapy about anything, they must have at least a modicum of energy and hope.

Dr. Richard Goldwasser, a courtly Loop psychologist who specializes in biofeedback techniques, saw a couple of clients from Sears Tower. He taught a middle-age woman who was an attorney on an upper floor to envision her favorite vacation spot just before going into the building each morning.

In the woman's imagination, the Loop at rush hour became a tranquil beach in Mexico.

Goldwasser taught another client to sit alone in his car in the parking garage each morning, taking deep breaths and practicing muscle-relaxation techniques before entering.

But there was something else going on, something other than fear. Graff had a longtime client who, immediately after Sept. 11, decided she needed to leave her husband and her lousy marriage.

Though she worked in the Loop, her husband had not called her to see if she was all right Sept. 11.

Graff observed that many people began to wonder: Who would grieve for me?

Who would be in all the newspapers holding up my photo?

Fearful of working in a high-profile skyscraper in a major American city, some people left jobs in Sears Tower without having other work lined up.

Nancy Kras, who left in late October after 20 years as a paralegal at Sonnenschein Nath & Rosenthal on the 80th floor, had no contingency plan.

Because Kras and her husband have no children, she felt like she could quit if she wanted to. And she wanted to. After two decades, she suddenly noticed how tall the building was as she returned to work on the train Sept. 13.

"I think," Kras said, "maybe the world is divided into two kinds of people: those who just go on no matter what, and those that don't."

Her departure left at least one colleague, de la Torre, with too much work. Feeling overwhelmed by the job and anxious about working on the 81st floor, de la Torre quit Dec. 28--though her husband kept his job as a computer-database analyst on the 34th floor.

Within weeks, de la Torre landed a new job, as a supervisor of paralegals at a firm in the 83-story Aon Center. Though she was in Chicago's second-tallest building, de la Torre felt safer because she was on a much lower floor, the eighth.

For Kras, leaving was more costly. She did not find work for almost a year. This month she starts a new job.

De la Torre is hiring her for a paralegal position.

Judie Mikuzis, Erin Frey and Erin Beavers, friends and co-workers at Heidrick & Struggles, would find peace in different places and in different ways.

Beavers, 31, a small-town girl from Pekin, Ill., once had delighted in having a job in the prestigious skyscraper. Her father told everyone he knew when she started working there.

Now she was one of several women at the executive search firm who wore sneakers all day every day so they could escape easily in an emergency--a club that Frey elected not to join lest she suffer from office politics for failing to wear the proper footwear.

A few days after Sept. 11, Beavers turned down a good job offer elsewhere. She loved working at Heidrick & Struggles. Her co-workers were like family, and the firm had a great reputation.

But Frey, 28, only stayed on in hopes of being among the rumored layoffs and getting a severance package. It was only a matter of time until she left, one way or another. Every decision she made now was rooted in the lessons of New York, which called for living as if you might die tomorrow.

Frey's personal journey after Sept. 11 would include a night at the Buzz Cafe, an Oak Park coffeehouse with hardwood floors and mismatched chairs.

Here, on a clear summer night in July, Frey would sing publicly for the first time with her new pop-rock band, Intuition, realizing a dream.

Well I've been afraid of changin', 'cause I built my life around you.

But time makes you bolder. Even children get older. And I'm gettin' older too.

Frey's boyfriend of 1 1/2 years, Jeff Sonsedek, demonstrated his grasp of the lessons of Sept. 11 by proposing. On Sept. 23, he gave Frey a ring as she stood on a karaoke stage, having just finished singing Shania Twain's "Any Man of Mine."

The ring was gold, and it had a 1 3/4-carat, marquis-cut diamond surrounded by 10 smaller diamonds.

Frey and Sonsedek had been planning for a day like this, had looked for rings in June.

Sept. 11 was the nudge toward the altar that they needed.

When Frey turned 29 on Oct. 2, she decided to start looking for a new job. She thought about her mortality, asking herself, "Why should I wait for someone to lay me off?"

Frey smokes half a pack a day, but when she's stressed out she smokes more, and her back kinks up. Staying at Sears Tower seemed unhealthy.

The first call she made was to someone her team at Heidrick & Struggles had placed in a job at Central DuPage Hospital. Frey's secret job search became her strength and sustenance.

At a regular, weekly team meeting in the boss' office in mid-October, she blurted, "Lay me off, please!"

But Frey knew what she was doing--she was zeroing in on that job at the hospital.

On Oct. 29 her phone rang.

It was a conference call from two of her bosses.

As you know, we're going through some hard times right now, and we need to pare down, they told Frey.

They were laying her off.

This is not a reflection of your work performance, they said.

But Frey, who got her severance, needed no consoling.

She was grinning.

Like her colleagues, Mikuzis had felt good when she started working at Sears Tower. She felt safe, having recently quit a risky job with the International Monetary Fund that required her to travel frequently to international hot spots.

But now she was so scared of working in the building that she couldn't sleep.

Mikuzis, 34, tried everything she could think of to ease her mind. Admittedly, some of her tactics seemed incongruous. Though she didn't watch the news for a week after Sept. 11, she got on the Internet and looked for whatever she could find about Sears Tower.

She drove to work every day rather than taking the train, because having the option of leaving quickly made her feel better.

She went out of her way to do things for other people because being helpful left little room for feeling helpless.

Long before she finally quit, Mikuzis was leaning toward leaving the tower--an inclination strengthened by the hijacking scare of Sept. 20. Yet something held her back.

Then, one morning in mid-October, she woke up and knew what to do.

Later that morning, when Tony Mikuzis called his wife at work, Judie told him what she had decided.

"I can't do this anymore," she said. "They're saying there's going to be another round of layoffs. How do you feel about my talking to upper management about taking voluntary severance?"

She and her husband had talked about having a baby. Sept. 11 had made Mikuzis want to go ahead. But she said she wondered: "If I'm this scared and stressed out all the time, we can't start a family; it won't happen. And if it does, then what if I'm pregnant and somebody puts chemicals in the air system of Sears Tower?"

Tony told her he didn't want her working there if she wasn't comfortable.

That was all Mikuzis needed to hear. She would waste no more time. She regretted the years lost from her relationship with Tony, who had been her high school sweetheart and college flame until she broke up with him over a rumor that he was cheating on her.

When they were reunited years later after failed marriages to others, Mikuzis discovered the rumor had not been true. But there's no regathering the years.

She turned to her computer and wrote her bosses an e-mail message in which she offered to save someone's job in return for a little peace of mind: freedom from Sears Tower.

Then she clicked SEND and breathed a sigh of relief.

On Oct. 29, the day former President George Bush visited Sears Tower to help reopen the Skydeck and the day Frey was laid off, Mikuzis entered her office, saw an envelope propped against her phone and smiled.

Inside the envelope was a letter directing her to meet with her bosses at 11 a.m. Her wish was being granted. After meeting with management, Mikuzis left the building, got in her car and drove away one last time, glancing back over her shoulder at the dark tower she once had thought so beautiful.

She launched a research and recruiting firm called 21st Century Research and Consulting Ltd. out of a bedroom in her home. And she became pregnant.

Some people asked how she could bring a baby into the world after what happened Sept. 11.

Mikuzis thought: How could you not? It's the best expression of love, to breathe new life into the world.

For nine months and a day, she and her husband didn't know whether the baby was a boy or a girl. They didn't want to know. But in the expectant mother's dreams, which had ceased being haunted by Sears Tower and now were all about her baby, the child was always a boy, with a face that resembled his dad's and toes that looked like hers.

On Aug. 24, a day after the due date, the baby arrived, weighing 8 pounds 12 ounces.

Sears Tower will never be the same. On the wall in the coffee room at Gronek & Armstrong on the 98th floor, there hangs an FBI advisory about what to do "if you receive a suspicious letter or package." For one thing, don't taste it, the poster warns.

Many women have sworn off high heels.

But in numerous other ways life at Sears has returned to normal. Women have stopped taking their purses with them wherever they go. E-mail about false alarms and security concerns has slowed. The attorney who had to imagine herself on vacation in Cancun so she could relax enough to enter the tower for work no longer sustains herself with flights of fancy.

The ranks of amateur photographers trying to preserve the tower for posterity have thinned.

At Heidrick & Struggles, consultants who had stopped flying after Sept. 11 are back in the sky.

When the law firm Schiff Hardin & Waite sought to hire someone for a marketing and communications job in April and May, about 800 people applied.

People who stayed at the tower had a wide spectrum of reasons for not leaving--the money was good, the lease was too favorable, the threat seemed overblown, the mini-van wasn't paid off. Had attorney Paul Wisniewski been a partner, he might have considered moving the firm. Short of that he had no other option. He couldn't afford to quit, and he didn't want to work for another firm.

For Nancy Dombrowski, the overriding motivation seemed to be defiance, which boiled over into outright anger after the hijacking scare of Sept. 20. But even those emotions were softening.

Because business was so slow after Sept. 11, she took nine days off at Christmastime and flew to Arizona with her boyfriend to meet family there and make a side trip to Las Vegas. It was the first time in 10 years Dombrowski had spent the holidays with family--and it was, in a twisted sort of way, all thanks to Sept. 11.

On Jan. 22, Dombrowski, now 33, learned she was pregnant.

In May she found out it was a boy.

The father, boyfriend Rick Chandler, wanted to name the boy Brett. But Dombrowski said it sounded too much like a character in a soap opera. She favors Collin. Or Payton, as in Walter, one of Chandler's heroes.

On a shelf in her office she has the book "15,000 Baby Names."

The due date, Sept. 27, is circled on her wall calendar.

Returning to her office after lunch on a bright afternoon in May, Dombrowski found a letter-size envelope on her chair. Inside was her new identification card for the building. Her photograph was on the front. She would need the card to gain access through the electronic turnstiles that were being installed in the lobby in response to what happened Sept. 11. Every little thing, it seemed, was a reminder.

After a few minutes, the phone in Dombrowski's office rang. It was a client in Florida.